Azeotropic composition



2,999,815 AZEOTROPIC CQMPOSITEON Bernhardt J. Eiseman, Jr., Wilmington,DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.,a corporation of Delaware No Drawing. Filed Aug. 11, 1960, Ser. No.48,817 1 Claim. (Cl. 252171) This invention relates to an azeotropiccomposition and particularly to the azeotropic mixture of1,1,2-trichloro- 1,2,2-trifluoroethane and acetone.

Several of the chlorofluoromethanes and chlorofluoroethanes haveattained widespread use as speciality solvents in recent years,particularly trichlorofluoromethane and trichlorotrifluoroethane. Bothof these compounds are relatively low boilingliquids (CCl F, 23.8 C.;CCI FCCIF 47.6 C.), which are nontoxic and nonflammable, and which havesatisfactory solvent power for greases, oils, waxes and the like. Theyhave therefore found widespread use for cleaning electric motors,compressors, oxygen storage tanks, photographic film, lithographicplates, typewriters, instruments, gauges, sound tape, and asnoncorrosive brines.

For certain solvent purposes however, the chlorofluoromethanes andchlorofluoroethanes have insufficient solvent power alone. This isparticularly true in the electronic industry during the manufacture ofprinted circuits. Printed circuits are well known in the electronicsart; and consist of a circuit formed from a soft metal on a solid,nonconducting surface such as a reinforced phenolic resin. Duringmanufacture, the solid surface is coated with the metal, the desiredportion of metal is coated with an impervious coating, and the excessmetal is removed by etching with a suitable acid. After the excess metalhas been removed, it is necessary to remove the impervious coatingbecause solder joints must be made to the printed circuit and these willnot form if the coating is present. After the impervious coating isremoved, the circuits are coated with a rosin flux to permit the jointsto be soldered, with rosin flux must then be removed. Thechlorofluoromethane and chlorofluoroethane solvents do not havesuflicient solvent power to clean printed circuits; that is, toelfectively remove the rosin flux.

It is an object of this invention to provide a novel azeotropiccomposition which has new and unusual properties. Another object is toprovide an azeotropic composition which is valuable as a solvent, andparticularly for cleaning printed circuits. A further object is toprovide such a composition which is nonflammable and nontoxic, both inthe liquid phase and in the vapor phase. Other objects are to providenew compositions of matter and to advance the art. Still other objectswill appear hereinafter.

The above and other objects may be accomplished in accord with thisinvention which comprises an azeotropic composition consisting of about87.5% by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and about 12.5%by weight of acetone and boiling at 45 C. at 760 mm. pressure.

It has been found that 1,1,2-trichloro-1,2,2-trifluoroethane andacetone, when admixed in the above proportions, form an azeotrope whichdistillls at constan t composition, the liquid phase and the vapor phasein equilibrium therewith having the same composition. Such mixture isnonflammable and nontoxic in both the liquid phase and the vapor phase.It is useful as a heat exchange medium and as a hydraulic fluid. It isparticularly useful as a solvent for greases, oils, waxes, and the like,in cleaning electric motors, compressors, photographic film, oxygenstorage tanks, lithographic plates, typewriters, precision instruments,gauges, sound tape,

vent properties of both components. so as to make the azeotrope anexcellent solvent mixture for such purposes, and does not deleteriously'aifect cellulose acetate cloth or laminated boards for printed circuits.

Cellulose acetate cloth (acetate rayon) is unusually susceptible todamage by solvents, particularly acetone, but is not alfectedbyl,1,Z-trichloro-1,2,24rifluorbethane. Good dry cleaning solvents arediificut to obtain for this fabric. To test the effect of the abovedescribed azeotrope on cellulose acetate fabric, a piece of the fabricwas immersed in the azeotrope overnight. It was then re.- moved anddried. The fabric was completelyu-naifected- When a similar piece offabric is allowed to stand in acetone overnight, it dissolvescompletely.

The azeotrope of this invention has several advan-" tages over thesolvents heretofore employed for cleaning printed circuits.Trichloroethylene, tetrachloroethylene, and methyl chloroform, areunstable and require stabilizers, whereas the azeotropic composition ofthis invention does not. The azeotropic composition of this invention ismore elfective as a solvent for such purposes than trichloroethylene andtetrachloroethylene. Inhibited methyl chloroform (containing dioxane asa stabilizer) is too strong a solvent and'attacks the backing of theprinted circuits. Isopropyl alcohol-toluene blends are flammable andirritating. The azeotropic mixture of1,1,2-trich1oro-1,2,2-trifluoroethane and acetone does not suffer fromany of these defects.

The azeotropic composition of this invention was obtained during thedistillation of two kilograms of a mixture containing 82.5% by weight of1,l,2-trichloro-l,2,2- trifiuoroethane and 17.5% by weight of acetonethrough a 1 inch x 38 inch column packed with /s inch glass helices at areflux ratio of 10/1 and a take off of 1 nil/min. The first fractionobtained was 1594.3 grams of the azeotrope containing 87.5% by weight of1,1,2- trichloro-l,2,2-trifluoroethane and 12.5% by weight of acetoneand having a boiling point of 45.0 C. at 760 mm. pressure. At thispoint, there was insuflicient 1,1,2- trichloro-1,2,2-trifluoroethaneremaining to maintain the azeotrope and nonazeotropic mixtures,containing much larger concentrations of acetone, distilled (169.7grams).

Printed circuit boards are usually prepared by impregnating glass cloth,nylon, or paper laminates with a phenol-formaldehyde resin or an epoxyresin. Printed circuits are prepared by a variety of methods. In atypical procedure, the board consists originally of a phenolic resinimpregnated base to which is bonded a sheet of copper, 2 to 4 milsthick, covering one surface of the board. The desired circuit is drawnon the copper with an asphalt based ink, using the silk screen method.The excess copper is then removed by etching with a ferricchloride-hydrochloric acid bath, sometimes containing ammonium chloride,leaving on the board the copper that is covered by the ink. Afterwashing off the etch solution, the asphalt ink is removed by cleaningwith the azeotropic composition of this invention in an-ultrasonic bath(some mechanical scrubbing is often used). The entire surface oftheboard is coated with a rosin flux and dried. The electronic components(resistors, capacitors, etc.) are then added at the proper places forsoldering to the circuit. The board is then passed over a molten solderbath, contacting the desired joints with the molten metal, whereby thesoldering is effected. After cooling, the excess rosin flux remaining onthe board must be removed since, if present in the final assembly, itwill lead to corrosion, poor electrical resistance and other deleteriousproperties.

The board is cleaned by placing it in an ultrasonic bath operating atabout 32 kilocycles per second and at about 25 C., where it remains forone minute. If 1,,1,2-trichloro-1,2,2-trifiuoroethane is used alone asthe solvent in the bath, it has no eifect on the rosin flux. Iftrichloroethylene is used as the solvent in the bath, about 5% to about50% of the flux remains after the treatment. If the same cleaningoperation is repeated on another circuit board using the azeotropicmixture of l,1,2-trichloro-1,2,2-trifluoroethane and acetone as thesolvent the rosin flux is completely removed. Neither the1,l,2-trichloro-l,2,2-trifluoroethane, nor the azeotropic mixture hasany eflect on the board which constitutes the backing of the printedcircuit.

It will be understood that the preceding detailed description of the useof the azeotropic composition of this invention in cleaning printedcircuits and cellulose acetate cloth is given for illustrative purposessolely, that this invention is not restricted to such specificembodiments and that other techniques may be employed. Also,

the azeotropic composition can be used for other purposes as indicatedin the general description.

From the preceding description, it will be apparent that this inventionprovides a novel azeotropic composition which has unusual properties andwhich is useful for a wide variety of purposes. Accordingly, it will beapparent that this invention constitutes a valuable contribution to andadvance in the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as' follows:

An azeotropic composition consisting of about 87.5% by weight ofl,1,Z-trichloro-1,2,2-trifluoroethane and about 12.5% by weight ofacetone and boiling at C. and 760 mm. pressure.

Industrial Solvents, Mellan, 2nd ed. (19S0),'pp. 48-79, 372, 373,582-589.

